The invention pertains to detection circuits and more specifically to integrated circuits that detect both AM and FM signals through sharing of circuit components for the two operations.
In particular, the invention provides improvement over the circuit disclosed in U.S. Pat. No. 3,999,138, entilted "Detector for AM-FM Signals," William Peil and Robert J. McFadyen, issued Dec. 21, 1976, in respect to bias and output voltage variations in AM and FM operation and dynamic range of the AM signal. While the noted patent discloses an AM-FM detector circuit of highly satisfactory operation well suitable for integrated circuit (IC) fabrication, unavoidable discrepancies in the processing of the IC's create differences in the electrical properties of the IC's active and passive components from one circuit to another, in particular the result of differences relating to geometry of the semiconductor components. These discrepancies, however slight, tend to produce bias and output voltage variations which reduce the percentage of circuits of the total number fabricated that have specifications falling within acceptable limits, commonly referred to as yield. In addition, in operation the patented circuit may exhibit additional bias and output signal variations with changes in ambient temperature. While these temperature related instabilities can be compensated for in FM operation by added voltage compensating circuitry, such compensation is not readily achieved in AM operation. This results in the output AGC voltage having a tendency to drift with temperature change, which requires a tightening of the specifications and thereby acts to further reduce the yield.